author's name gets link only if AUTHORS_SAVE_AS is set.

[mstreuhofer]

There seems to be no point in linking to the author's index page listing all articles of that author if there is only one author. If the pelican configuration states that no overview page of all authors should be generated we can assume only one author exists and thus not link the author's name in the article footer.

[mstreuhofer]

62 introduced that the author's name gets linked in the article's footer. this PR here extends on that logic but only creates a link if the pelican configuration does not explicitly state that no all-authors-overview is required (which is understood as only one author exists - thus no need to link the author's name).

[duilio]

Thanks!